Latching devices for the actuator assemblies of hard disk drives are employed in those drives in which the actuator assembly is moved to a parked position on the disk(s) with the transducer(s) engaging the surface of the respective disks in a landing or parking zone removed from the data fields. The parking zone is usually located adjacent the hub of a disk. In this parking zone position of the actuator assembly, a latch lever of the latching device is biased in a direction to engage and latch the actuator assembly.
These latching devices have taken various forms all of which have had a common goal, namely requiring no electrical power or minimal electrical power for operation. The more common actuator assembly latching devices which latch the actuator assemblies in a transducer parking zone on the disks may be divided into three general classes:
One is the air vane operated type of latch device seen in such U.S. Patents as U.S. Pat. Nos. 4,538,193; 4,647,997; 4,692,829 and 5,036,416.
A second class is the mechanically or magnetically restrained, actuator force released type of latch device. Actuator release occurs when the actuator motor is energized to move the actuator out of parked position. This type of latch device is seen in U.S. Pat. Nos. 4,562,500; 5,003,422; 5,023,736; 5,117,318 and 5,187,627.
A third is the spring engaged electromagnetically released type of latch device represented in such U.S. Patents as U.S. Pat. Nos. 4,796,130, 4,965,684; 5,012,371 and a publication EP-411-552-A.
With regard to the first type of latch device, as disk drives decrease in size, airflow volume rates and air vane areas are reduced to a point where the aerodynamic force acting on the air vane is insufficient to release the latch. Structural fragility is also a problem.
With regard to the second type of latch device, as disk drives decrease in size, the actuator motors are reduced in size and the actuator forces available to release the actuator from the grip of the fixed bias latch diminish. Here, a point is reached where actuator motor forces are insufficient for latch release.
With regard to the third type of latch device, electrical power is required for operating the latch. A fixed latching bias, as by a spring or a permanent magnet, engages the latch lever with the actuator assembly when the actuator assembly is in parked position. In such a design, the coil of the electromagnet is energized to move the latch lever to disengage the actuator assembly. The coil of the electromagnet must remain energized to hold the latch lever in actuator released position throughout the period of operation of the disk drive. This involves a continuous use of electrical power in the disk drive which is undesirable. On the other hand, the electromagnetically operated latch device appears to be the only viable latch device among those considered herein for use in small form factor disk drives, but those disclosed in U.S. Pat. Nos. 4,796,130 and 5,012,371 as well as European Publication EP-411-552-A, while employing electromagnetically actuated latches are not well suited for application in small form factor disk drives.
A latch lever 19 in U.S. Pat. No. 4,796,130 is spring biased to latched position and moves from latched position against a spring bias when the solenoid 21 in energized.
A latch lever 54 in U.S. Pat. No. 5,012,371 is moved between latched and unlatched positions by solenoid 62. Spring biasing of the latch lever 54 in latched position is not mentioned; only that the movable member 70 of the solenoid has two positions.
In publication EP-411-552-A a torsion spring biased lever 7 is rotated by an arm 3a of the rotary actuator upon movement to parked position where the arm 3a is captured. The spring biased armature 5a of a solenoid 6a moves below the distal end of the latch lever 7 to secure a latch lever in latched position. When the solenoid 6a is energized the solenoid armature is withdrawn which releases the lever 7 which is torsion spring biased to released position.
All three of the references aforesaid use linear stroke solenoids and are mechanically unsuited for down sizing for design integration in a disk drive.
In U.S. Pat. No. 4,965,684 one end 104, the armature end, of a pivotally mounted lever 38 confronts the pole end of a solenoid 36 defining a variable gap. The other end, the latch end, of the pivotally lever 38 mounts a pin 96 which engages a notch 98 in the rotary actuator coil housing 42. This latches the actuator in parked position. When the coil of the solenoid 36 is energized the armature 104 is attracted toward the pole end of the solenoid 36 removing the pin 96 from the notch 98, releasing the actuator for movement. Here, again, mechanical complexity negates scaling down of the design.
Another configuration of an electromagnetic latch, for an actuator in a disk drive, comprises a U-shaped yoke which mounts a coil. The ends of the two pole legs of the U-shaped yoke define pole tips., A pivotally mounted latch lever has a pole arm which mounts an armature or keeper. The latch lever is pivotally mounted on a pivot to rotate about an axis centered in a position above the pole tip of a first one of the two pole legs. In this position, the keeper bridges the pole legs, defining a magnetic gap with each of the pole tips, the magnetic gap between the pole tip at the end of the second one of the two pole legs of the yoke and the distal end of the keeper, being a variable length magnetic gap. The latch lever is pivotally mounted to support the keeper for angular movement between a first angular position, with the coil energized, in which the variable length gap is of minimal dimension and a second angular position, namely the spring biased actuator latched position, when the coil is de-energized. In this actuator latched position, the variable length gap is at its maximum dimension.
The larger the length dimension of the variable air gap when the latch lever is in actuator latched position, the higher the reluctance of the magnetic circuit. The latch lever has a latch arm which mounts a latch member. The latch member engages the actuator assembly when the actuator assembly is in parked position. The magnitude of the displacement of the latch member between actuator latched and actuator unlatched positions, to achieve positive actuator latching and unlatching, is a controlling factor in the design of an electromagnetic latch for a disk drive actuator.
This design of an electromagnetic latch is lacking in functional efficiency and effectiveness in at least two respects. The first is that the flux coupling at the magnetic gap at the first leg, being centered on the keeper at the axis of rotation of the latch lever, produces no useful torque about that axis. The second is that, in the actuator latched position of the latch lever, the magnetic gap at the distal end of the keeper is large and the reluctance of the magnetic circuit is high, requiring a high coil current to move the latch lever from the latched position, which is undesirable. Keeping the gap at the distal end of the keeper small, limits angular movement of the latch lever, necessitating a long length of the latch lever of the latch arm to perform the latching function. Making the latch arm of the latch lever longer than the pole arm may provide adequate displacement of the latch member at the end of the latch arm with reduced angular displacement of the latch arm for latching and releasing purposes, but this presents other problems, such as space accommodation, when an attempt is made to scale down the electromagnetic latch for structural integration in small form factor disk drives.
Having the magnetic gap at the distal end of the keeper close to the axis of the latch lever, minimizes the dimension of that magnetic gap in the latched position of the latch lever, but the electromagnetic moment arm, being short requires higher flux densities in the gap at the distal end of the keeper to achieve the required torque for operating the latch lever.
The primary disadvantage of this design is that it is difficult to scale the electromagnetic latch to a smaller disk drive. If the distance of travel of the latch end of the latch lever is fixed, an even greater difference between the electromagnetic moment arm and the latch moment arm is required if the overall size is to be reduced.
A disk drive having a structurally integrated electromagnetic latch is needed in which increased angular displacement of the latch member is achieved in a device of reduced size, in which the magnetic circuit reluctance is low, and in which the variation of the reluctance of the magnetic circuit between latched and unlatched angular positions of the latch lever is minimized, to provide positive latching and unlatching of the disk drive actuator assembly while minimizing operating current requirements.